Topics

Baffle question


weems@...
 

Today I took the focuser off of my 6" f9, getting ready to call AP and then send it and ADA2003A in to have the threads matched. I decided to pull the cell out too and give the tube a good cleaning. In 36 years of owning this, I've never really given the baffling any thought, but while I was going over it, I became curious. The tube has just one baffle, about 27" in from the rear element, and the hole (as best I could tell, reaching in with a bent tape measure) is 3" in diameter. To me, this seems small for fully illuminating a 2" wide field eyepiece. Half the objective diameter at half the focal length sounds like it would give a fairly small unvignetted circle. 

Am I wrong about this? Or should I be considering a change to the baffle? If nothing else, it needs repainting - the ring between the AP-8 casting and the tube is down to mostly shiny aluminum. 

I also note that more recent scopes use more than a single baffle. 

Chip Weems


thefamily90 Phillips
 

Hi Chip,
I don’t know the answers to your questions but I’ll bet I know who does. If you’re shipping it to Astro-physics just ask Roland to look at it and see if he would recommend anything. AP is an amazing company as you know. They will be happy to help you.

Best

Jim


From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of weems@... <weems@...>
Sent: Sunday, April 4, 2021 11:07:18 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>
Subject: [ap-ug] Baffle question
 
Today I took the focuser off of my 6" f9, getting ready to call AP and then send it and ADA2003A in to have the threads matched. I decided to pull the cell out too and give the tube a good cleaning. In 36 years of owning this, I've never really given the baffling any thought, but while I was going over it, I became curious. The tube has just one baffle, about 27" in from the rear element, and the hole (as best I could tell, reaching in with a bent tape measure) is 3" in diameter. To me, this seems small for fully illuminating a 2" wide field eyepiece. Half the objective diameter at half the focal length sounds like it would give a fairly small unvignetted circle. 

Am I wrong about this? Or should I be considering a change to the baffle? If nothing else, it needs repainting - the ring between the AP-8 casting and the tube is down to mostly shiny aluminum. 

I also note that more recent scopes use more than a single baffle. 

Chip Weems

--
Jim Phillips


Len Fulham
 

Chip,

A lot of scopes especially older ones use a single baffle.The baffle is usually dimensioned to fully illuminate a small central area of the field where critical viewing would be done. This generally suits visual scopes and does not cause issues. The single baffle is then very effective in blocking stray light.

Scopes which are optimised for imaging allow for a wider fully illuminated field. To achieve this a single baffle cannot effectively stop stray light so multiple baffles become necessary (unless you use a overly fat tube!). This is evident when one looks into a modern AP scope with numerous quite wide baffles. They are not there for show.

If you wish to revise the scope baffles you should consider the implications. Just making the single baffle opening larger will reduce contrast and may cause you problems. There is simple baffle calculator here:

http://www.myoptics.at/atmlj/stuff/bafflecalc.html

This will let you play with the parameters and get idea what changes you need to make to have a larger fully illuminated field. Eventually the opening of the focuser tube may become restrictive. It is very helpful to draw (on computer is OK) a proper scaled layout of your scope to examine the possible reflection paths between the objective and the field.

As a practical example I have a Brandon 94mm APO which has a single "mid tube" baffle. It fully illuminates about 3mm of the field. I have designed a set of 4 baffles to allow full illumination of a 40mm field. A lot more work, weight in scope etc. Makes for an interesting project.

Cheers,

Len.


weems@...
 

Hi Jim,

I doubt that AP would be able to do much for me short of making an entirely new tube assembly. This is a very old cell design that fits a 7" OD tube, and the single baffle isn't movable. Maybe the casting could be ground out by hand to a larger diameter, but it's pretty far in to reach with a mill.

Thanks Len,

According to the baffle calculator, the existing baffle will give a 1mm illuminated circle. It would actually place the baffle about 53mm further forward and 6mm wider, but on my full size drawing, they lay on the same rays. 

For a 47mm circle, it recommends 3 baffles, with the one near the middle about an inch wider. The rear one is very close to the position and opening diameter of the front face of the focuser. And there is one thin baffle about 10" in from the cell. 

My eyepieces have field stops ranging from 8mm to 47mm.

Chip


ROBERT WYNNE
 

Methinks your tube is a precision aluminum tube in which the inner diameter was originally bored on a lathe. A large boring bar would easily cut out the old single baffle and new baffles could be turned on a lathe. It's an easy job if you have the equipment. -Best, Robert

On 04/05/2021 6:52 PM weems@... wrote:
 
 
Hi Jim,

I doubt that AP would be able to do much for me short of making an entirely new tube assembly. This is a very old cell design that fits a 7" OD tube, and the single baffle isn't movable. Maybe the casting could be ground out by hand to a larger diameter, but it's pretty far in to reach with a mill.

Thanks Len,

According to the baffle calculator, the existing baffle will give a 1mm illuminated circle. It would actually place the baffle about 53mm further forward and 6mm wider, but on my full size drawing, they lay on the same rays. 

For a 47mm circle, it recommends 3 baffles, with the one near the middle about an inch wider. The rear one is very close to the position and opening diameter of the front face of the focuser. And there is one thin baffle about 10" in from the cell. 

My eyepieces have field stops ranging from 8mm to 47mm.

Chip


weems@...
 

Thanks Robert,

It's possible that the tube was bored on a lathe, but it also matches the dimensions of Hastings 7" tubing, and it's not perfectly round. If I had a way to mill the ends precisely perpendicular, and exactly match the placement of the threaded holes for the set screws that hold the cell and focuser, to preserve the collimation, then I would consider replacing the tube. 

I agree that a lathe large enough to hold a tube 44" by 7" with a boring bar long enough to reach 17" into it would be a way to remove the existing baffle. But then that could also be accomplished with a reciprocating saw. I think it would be preferable to just enlarge the opening in the casting to work with the light cone for the new baffles. I suspect, given how thin the tube is, and how thick the casting is, that part of its purpose was to ensure that the tube won't be crushed in the middle by over-tightening mounting rings.

Chip


ROBERT WYNNE
 

On thin cross sections take small cuts until down to parent material. The tube absolutely requires a perfect dial indicator "dial in" to ensure perfect alignment on the tubes length on all axis's. Not being perfectly round on the OD may be a problem. But I suspect the ID if bored is likely truer than the OD. Getting the tube fixtured so that the ID runs true should not be so much of a problem. Plus I would chuck the tube as close to the baffle as possible so that the chuck and the baffle are closely coupled during boring. In this way you may actually squeeze the tube into round. I would dial in the ID as close to perfection as possible and ignore the OD runout so that a re-bored ID has no mis-match through the tubes length. This is all dependent on the rigidity of the original tube. It's possible that residual stresses may be unlocked by removing the baffle and result in a out or round condition after a re-bore. You don't mention wall thickness which is a prime concern in any post machining of the tube. -Best, Robert

On 04/06/2021 6:31 AM weems@... wrote:
 
 
Thanks Robert,

It's possible that the tube was bored on a lathe, but it also matches the dimensions of Hastings 7" tubing, and it's not perfectly round. If I had a way to mill the ends precisely perpendicular, and exactly match the placement of the threaded holes for the set screws that hold the cell and focuser, to preserve the collimation, then I would consider replacing the tube. 

I agree that a lathe large enough to hold a tube 44" by 7" with a boring bar long enough to reach 17" into it would be a way to remove the existing baffle. But then that could also be accomplished with a reciprocating saw. I think it would be preferable to just enlarge the opening in the casting to work with the light cone for the new baffles. I suspect, given how thin the tube is, and how thick the casting is, that part of its purpose was to ensure that the tube won't be crushed in the middle by over-tightening mounting rings.

Chip


thefamily90 Phillips
 

Is it that expensive for a new tube?

JimP 


From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of ROBERT WYNNE <robert-wynne@...>
Sent: Tuesday, April 6, 2021 3:57:58 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>; weems@... <weems@...>
Subject: Re: [ap-ug] Baffle question
 
On thin cross sections take small cuts until down to parent material. The tube absolutely requires a perfect dial indicator "dial in" to ensure perfect alignment on the tubes length on all axis's. Not being perfectly round on the OD may be a problem. But I suspect the ID if bored is likely truer than the OD. Getting the tube fixtured so that the ID runs true should not be so much of a problem. Plus I would chuck the tube as close to the baffle as possible so that the chuck and the baffle are closely coupled during boring. In this way you may actually squeeze the tube into round. I would dial in the ID as close to perfection as possible and ignore the OD runout so that a re-bored ID has no mis-match through the tubes length. This is all dependent on the rigidity of the original tube. It's possible that residual stresses may be unlocked by removing the baffle and result in a out or round condition after a re-bore. You don't mention wall thickness which is a prime concern in any post machining of the tube. -Best, Robert
On 04/06/2021 6:31 AM weems@... wrote:
 
 
Thanks Robert,

It's possible that the tube was bored on a lathe, but it also matches the dimensions of Hastings 7" tubing, and it's not perfectly round. If I had a way to mill the ends precisely perpendicular, and exactly match the placement of the threaded holes for the set screws that hold the cell and focuser, to preserve the collimation, then I would consider replacing the tube. 

I agree that a lathe large enough to hold a tube 44" by 7" with a boring bar long enough to reach 17" into it would be a way to remove the existing baffle. But then that could also be accomplished with a reciprocating saw. I think it would be preferable to just enlarge the opening in the casting to work with the light cone for the new baffles. I suspect, given how thin the tube is, and how thick the casting is, that part of its purpose was to ensure that the tube won't be crushed in the middle by over-tightening mounting rings.

Chip

--
Jim Phillips


ROBERT WYNNE
 

Could be depending on the shop.
 
The engine lathe required is large and not normally a standard lathe in a small/medium size shop. The boring bar is relatively large and requires a machinist experienced with both.
 
If you go into a machine shop and start off speaking about precision telescope components when all you want is to clean up the baffle with a #16 surface finish then you might unintentionally bump up a quote quite a bit. Personally I wouldn't even mention it's part of a telescope to a shop.
 
Just say you want to machine out a clear bore through the tube as precisely as economical. The shop might just treat the job as another pipe and the quote should be lower than if quoting a precision telescope component.
 
Re-machining new baffles is an area I'm unfamiliar. If someone could explain the geometry of a baffle and the precision with which one has to be fabricated I would know better on how to submit for the best quote. I am a intrigued how a replacement baffle is intended to be coupled to the inner diameter of a tube. I doubt it is a press fit?
 
Best,
Robert

On 04/06/2021 1:33 PM thefamily90 Phillips <thefamily90@...> wrote:
 
 
Is it that expensive for a new tube?
 
JimP 
 

From: main@ap-ug.groups.io <main@ap-ug.groups.io> on behalf of ROBERT WYNNE <robert-wynne@...>
Sent: Tuesday, April 6, 2021 3:57:58 PM
To: main@ap-ug.groups.io <main@ap-ug.groups.io>; weems@... <weems@...>
Subject: Re: [ap-ug] Baffle question
 
On thin cross sections take small cuts until down to parent material. The tube absolutely requires a perfect dial indicator "dial in" to ensure perfect alignment on the tubes length on all axis's. Not being perfectly round on the OD may be a problem. But I suspect the ID if bored is likely truer than the OD. Getting the tube fixtured so that the ID runs true should not be so much of a problem. Plus I would chuck the tube as close to the baffle as possible so that the chuck and the baffle are closely coupled during boring. In this way you may actually squeeze the tube into round. I would dial in the ID as close to perfection as possible and ignore the OD runout so that a re-bored ID has no mis-match through the tubes length. This is all dependent on the rigidity of the original tube. It's possible that residual stresses may be unlocked by removing the baffle and result in a out or round condition after a re-bore. You don't mention wall thickness which is a prime concern in any post machining of the tube. -Best, Robert
On 04/06/2021 6:31 AM weems@... wrote:
 
 
Thanks Robert,

It's possible that the tube was bored on a lathe, but it also matches the dimensions of Hastings 7" tubing, and it's not perfectly round. If I had a way to mill the ends precisely perpendicular, and exactly match the placement of the threaded holes for the set screws that hold the cell and focuser, to preserve the collimation, then I would consider replacing the tube. 

I agree that a lathe large enough to hold a tube 44" by 7" with a boring bar long enough to reach 17" into it would be a way to remove the existing baffle. But then that could also be accomplished with a reciprocating saw. I think it would be preferable to just enlarge the opening in the casting to work with the light cone for the new baffles. I suspect, given how thin the tube is, and how thick the casting is, that part of its purpose was to ensure that the tube won't be crushed in the middle by over-tightening mounting rings.

Chip

--
Jim Phillips


Len Fulham
 

Chris,
Your AP 6" is a similar vintage to mine. AP Scopes of that era are relatively primitive when compared to the more recent AP product. (Still V good tho). I am pretty sure the tubes were hand cut and file finished square. There is no indication the tubes were lathe turned on either surface. The tubes tend to be out of round to a similar degree to standard aluminium irrigation pipe which I have used for scope tubes.

I believe the baffle is turned to size and pushed into the tube. I do not know how Roland got the baffles in & fixed, but when I needed to do a similar job on a 110mm refractor, I lubricated the interior of the tube with ordinary soap and then pushed the baffles in with a marked stick. I then hosed the soap out and painted the tube. No glue or fastener - the paint and friction are enough to ensure the baffles will never move.

In some scopes (eg Brandon 94) the one baffle was a loose fitting pressed steel affair like half a tin can with cuts in the wall and the corners bent out to grab on the tube walls - so loose and flimsy that they could possible move with harsh handling. (Not all scopes are equal!)

It would be worth asking at AP how the baffle is fixed and prospect of relocating it. If all else fails you could use a compact Festool or similar oscillating saw to cut (split) the baffle so it can be removed, then make new ones as required.

Cheers,

Len.


ROBERT WYNNE
 

These scopes were probably bored, chucked on the ID, from raw aluminum tube stock which is close but not perfect. As I recall raw stock always allows at least .005 - .010" extra material, maybe more, to cut past any inclusions to parent metal.
 
The light path being the critical driver in this design requires a very precise linear ID bore with the OD likely left in raw stock condition, there being no reason to invest further time as far as form, fit or function.
 
I seriously doubt the ends were filed flat especially when the tube was already chucked in the lathe. It would be a simple 30 second pass with a cut off tool to bring the ends square and flat. I believe there is additional associated machining on both ends that require the use of a lathe and then there's the problem of cutting the tube to its proper length.
 
As far as a press fit for the baffle goes, unless the baffle was "sweat fit" into position it's doubtful that it would meet a factory permanent seating requirement. Just a change in temperature could cause a shift in its position.
 
Affixing the baffle into a position square to the ID may also require a special fixture to seat precisely.
 
I would not recommend removing a baffle by cutting as the risk of cutting into the tube's main ID is one I would not wish to take on. If the baffle merely presses out that's another story and the easiest route to solving this problem. I would check with George at A-P for the correct solution.
 
I believe we may have similar scopes of the same vintage. Mine is a 5" F8 Starfire.
 
-Best, Robert

On 04/06/2021 3:28 PM Len Fulham <lfulham@...> wrote:
 
 
Chris,
Your AP 6" is a similar vintage to mine. AP Scopes of that era are relatively primitive when compared to the more recent AP product. (Still V good tho). I am pretty sure the tubes were hand cut and file finished square. There is no indication the tubes were lathe turned on either surface. The tubes tend to be out of round to a similar degree to standard aluminium irrigation pipe which I have used for scope tubes.

I believe the baffle is turned to size and pushed into the tube. I do not know how Roland got the baffles in & fixed, but when I needed to do a similar job on a 110mm refractor, I lubricated the interior of the tube with ordinary soap and then pushed the baffles in with a marked stick. I then hosed the soap out and painted the tube. No glue or fastener - the paint and friction are enough to ensure the baffles will never move.

In some scopes (eg Brandon 94) the one baffle was a loose fitting pressed steel affair like half a tin can with cuts in the wall and the corners bent out to grab on the tube walls - so loose and flimsy that they could possible move with harsh handling. (Not all scopes are equal!)

It would be worth asking at AP how the baffle is fixed and prospect of relocating it. If all else fails you could use a compact Festool or similar oscillating saw to cut (split) the baffle so it can be removed, then make new ones as required.

Cheers,

Len.


ROBERT WYNNE
 

Meant chucked on the OD. -Best, Robert

On 04/06/2021 5:51 PM ROBERT WYNNE <robert-wynne@...> wrote:
 
 
These scopes were probably bored, chucked on the ID, from raw aluminum tube stock which is close but not perfect. As I recall raw stock always allows at least .005 - .010" extra material, maybe more, to cut past any inclusions to parent metal.
 
The light path being the critical driver in this design requires a very precise linear ID bore with the OD likely left in raw stock condition, there being no reason to invest further time as far as form, fit or function.
 
I seriously doubt the ends were filed flat especially when the tube was already chucked in the lathe. It would be a simple 30 second pass with a cut off tool to bring the ends square and flat. I believe there is additional associated machining on both ends that require the use of a lathe and then there's the problem of cutting the tube to its proper length.
 
As far as a press fit for the baffle goes, unless the baffle was "sweat fit" into position it's doubtful that it would meet a factory permanent seating requirement. Just a change in temperature could cause a shift in its position.
 
Affixing the baffle into a position square to the ID may also require a special fixture to seat precisely.
 
I would not recommend removing a baffle by cutting as the risk of cutting into the tube's main ID is one I would not wish to take on. If the baffle merely presses out that's another story and the easiest route to solving this problem. I would check with George at A-P for the correct solution.
 
I believe we may have similar scopes of the same vintage. Mine is a 5" F8 Starfire.
 
-Best, Robert
On 04/06/2021 3:28 PM Len Fulham <lfulham@...> wrote:
 
 
Chris,
Your AP 6" is a similar vintage to mine. AP Scopes of that era are relatively primitive when compared to the more recent AP product. (Still V good tho). I am pretty sure the tubes were hand cut and file finished square. There is no indication the tubes were lathe turned on either surface. The tubes tend to be out of round to a similar degree to standard aluminium irrigation pipe which I have used for scope tubes.

I believe the baffle is turned to size and pushed into the tube. I do not know how Roland got the baffles in & fixed, but when I needed to do a similar job on a 110mm refractor, I lubricated the interior of the tube with ordinary soap and then pushed the baffles in with a marked stick. I then hosed the soap out and painted the tube. No glue or fastener - the paint and friction are enough to ensure the baffles will never move.

In some scopes (eg Brandon 94) the one baffle was a loose fitting pressed steel affair like half a tin can with cuts in the wall and the corners bent out to grab on the tube walls - so loose and flimsy that they could possible move with harsh handling. (Not all scopes are equal!)

It would be worth asking at AP how the baffle is fixed and prospect of relocating it. If all else fails you could use a compact Festool or similar oscillating saw to cut (split) the baffle so it can be removed, then make new ones as required.

Cheers,

Len.


weems@...
 

Len, 

I agree that this looks like irrigation pipe. The wall thickness is uniform, but there is about a 0.1" variation in the diameter. I held the tube up to a light and can see gaps around the baffle in a couple of places. Using a long dowel, going around the perimeter, tapping lightly with a hammer, I was able to push it back about 1/4" in 1/16" increments. I think it is taking advantage of the irregularity of the tube to act in compression, for holding it in place.

I may be able to come up with a combination of field illumination and placement that will allow me to just move it back. Using the Berfield approach of tracing rays, it looks like a 35mm illuminated field would put it at about the right place and diameter for the second baffle, if moved back around 6".

This is a January '85 NASA glass 6" f9, so it predates the Starfire. That was a time when AP was first moving from selling objectives for DIY builders, to providing a complete OTA. Mine originally came with a temporary Jaegers style focuser, which was replaced a few months later by the first of the 2.7" ones. Unfortunately the early focusers had a slightly different thread diameter, so the current tailpiece doesn't fit (which is where this whole saga began, with me pulling the focuser, so it could be sent in). 

Chip


ROBERT WYNNE
 

Doubtful irrigation pipe. More likely: SCHEDULE 40 ALUMINUM PIPE 6061-T6-EXTRUDED given A-P's predisposition to perfection on materials used in their products even in the old days. Today A-P advertises "virgin aluminum material" and that concept likely began at the start of the company. Do you mean the ID has a total run out of .1"? If so what about the ID? If the ID has .1" run out, then there would be huge gaps between baffle and tube ID. BTW What is the wall thickness? -Best Robert

On 04/06/2021 7:23 PM weems@... wrote:
 
 
Len, 

I agree that this looks like irrigation pipe. The wall thickness is uniform, but there is about a 0.1" variation in the diameter. I held the tube up to a light and can see gaps around the baffle in a couple of places. Using a long dowel, going around the perimeter, tapping lightly with a hammer, I was able to push it back about 1/4" in 1/16" increments. I think it is taking advantage of the irregularity of the tube to act in compression, for holding it in place.

I may be able to come up with a combination of field illumination and placement that will allow me to just move it back. Using the Berfield approach of tracing rays, it looks like a 35mm illuminated field would put it at about the right place and diameter for the second baffle, if moved back around 6".

This is a January '85 NASA glass 6" f9, so it predates the Starfire. That was a time when AP was first moving from selling objectives for DIY builders, to providing a complete OTA. Mine originally came with a temporary Jaegers style focuser, which was replaced a few months later by the first of the 2.7" ones. Unfortunately the early focusers had a slightly different thread diameter, so the current tailpiece doesn't fit (which is where this whole saga began, with me pulling the focuser, so it could be sent in). 

Chip


Len Fulham
 

Chip, Robert,
In my case the 6” f9 scope and 706 mount (https://ap-ug.groups.io/g/main/photo/113262/1364286?p=Name,,706,20,1,0,0) are in our club observatory and mostly used for  visual observing/outreach so the fact they may not be photo optimised is not an issue.  The 6" scope tube is as-found and has been surface prepped and nicely painted externally and painted black internally. It is concievable that the tube has been mounted on a mandrel and the ends turned square, but the finish looks more consistent with hand cutting/finishing. The lens is a coma free design so precise axial alignment is not necessary. Interestingly the original tube mounting rings were one piece split on one side with a single locking bolt - they were a neat sliding fit on the tube and lined with thin plastic tape; when tightened they could be seen to take the small irregularities out of the tube.
 
The lens cell is a simple loose push fit in the tube end and secured by three small grub screws pressing on the cell; the cell is not drilled or tapped, The dewcap is a similar slip on piece of aluminium tube, no fixings.
The focuser also pushed in and was retained by similar grub screws. Eventually I made a new back plate with tapped retaining screws through the original grub screw holes - note these holes are ‘about’ 120˚ apart - only line up one way. The new backplate was threaded to accept the then new A-P made (?) 2.7” focuser.
 
The next AP scope I purchased was the 178 f9 Starfire (about 1989). By then Roland had improved production methods and was using in house machining more. The fit and finish of the 7” scope is to a much higher standard than the earlier 6” eg the lens cell is adjustable, the dew cap threads on and has a machined front ring; the back plate is fully machined and threaded to take the focuser (instead of being integral). The tube contains four baffles, each is a one piece pressed metal sleeve and ring. Light and effective but the rear baffle has actually shifted about 10mm so clearly not fixed aggressively.  Attached is a photo of the baffle which has slipped - you can see that it does not have conformity with the tube walls - so it is not a  tight fit, as evidenced by the fact that it has moved. I suspect that friction and paint are all that is (isn’t) retaining it. Robert your scope is probably from around that time also. (EDF/EDT appellations came a little later with some overlap).
 
The later products with fully machined tubes are elegant and beautiful constructs, but there is a weight penalty. For their size the modern scopes eg my 130 EDF GT and 106 Traveler are quite hefty.
 
Going back to the scope I made, the baffles were machined from solid bar stock 140mm dia with a solid ring 10mm thick x 25mm long with the baffle at the front end. The insert edge of the ring was slightly tapered to assist entry to the tube. Dry they were almost impossible to insert into the tube - certainly felt like they would be impossible to get out again - but with a nice slime of soap they pushed in without a lot of effort, making the tube round in doing so. The straight turned sides of the baffle ensured they were precisely square to the tube.
 
The products A-P make now are superlative but not perfect; in the past they were less perfect :-}In their day, they were excellent, and still are.

Cheers,

Len.


Len Fulham
 

And the attachment!   7" Baffle


ROBERT WYNNE
 

Is the OD offset to the ID to a common centerline as depicted in the photo. The walls of the tube are not of equal width? Or is that an artifact from the off axis POV of the camera's lens? -Best, Robert

On 04/06/2021 9:59 PM Len Fulham <lfulham@...> wrote:
 
 
And the attachment!   7" Baffle


Len Fulham
 

Robert,
"is that an artifact from the off axis POV of the camera's lens?"
The baffles are centered along the tube axis as would be expected. I purposefully took the image off axis to allow the individual baffles to be seen.
Cheers,

Len.


weems@...
 

Len,

Those are very nice knife-edge baffles in the Starfire. I'd like to understand why you chose a 40mm field for your Brandon. There seem to be a wide range of opinions on the diameter of the field. Some people say that 25mm is plenty, and will yield a visual falloff of only half a magnitude on a 2" wide field eyepiece. Some argue for even smaller, to improve contrast in the central area at higher powers. Others say to go bigger to get even illumination over a wide field. When I ray trace for smaller than 30mm, I start to see systems that would put a stop at a distance that would be inside the focuser tube, smaller than its diameter. My scope is mounted side by side with a C14, so I use it mainly for objects that exceed the narrow view of the 14, but I also crank up the power on nights when the seeing is too rough for the larger aperture. 

I took a few photos to show what I'm working with. The first one shows the baffle. It's a casting that is mounted in a piece of aluminum plate. At the edge of the hole, it's about 1/4" thick. Certainly not a sharp edge. 



The next one shows the gap on one side. I couldn't get the other gap illuminated at the same time, but it's similar. 



The third one is the edge of the tube at the objective end. The wall thickness is uniform, but the diameter varies from 172mm to 177mm.



Here's the cell, showing the Jan. 85 date and 614.



Chip


ROBERT WYNNE
 

In photo #3 you state, The third one is the edge of the tube at the objective end. The wall thickness is uniform, but the diameter varies from 172mm to 177mm. Do you intend the ID or the OD? -Best, Robert

On 04/08/2021 7:02 PM weems@... wrote:
 
 
Len,

Those are very nice knife-edge baffles in the Starfire. I'd like to understand why you chose a 40mm field for your Brandon. There seem to be a wide range of opinions on the diameter of the field. Some people say that 25mm is plenty, and will yield a visual falloff of only half a magnitude on a 2" wide field eyepiece. Some argue for even smaller, to improve contrast in the central area at higher powers. Others say to go bigger to get even illumination over a wide field. When I ray trace for smaller than 30mm, I start to see systems that would put a stop at a distance that would be inside the focuser tube, smaller than its diameter. My scope is mounted side by side with a C14, so I use it mainly for objects that exceed the narrow view of the 14, but I also crank up the power on nights when the seeing is too rough for the larger aperture. 

I took a few photos to show what I'm working with. The first one shows the baffle. It's a casting that is mounted in a piece of aluminum plate. At the edge of the hole, it's about 1/4" thick. Certainly not a sharp edge. 



The next one shows the gap on one side. I couldn't get the other gap illuminated at the same time, but it's similar. 



The third one is the edge of the tube at the objective end. The wall thickness is uniform, but the diameter varies from 172mm to 177mm.



Here's the cell, showing the Jan. 85 date and 614.



Chip